First off, this is Southern California. Water that falls from the sky is considered a flaw in nature. When that water gets all hard and crunchy thats just plain evil. The snow at Disneyland is as close as any self -respecting Southern California will get. We put snow on the mountains to make the television pictures of the Rose Parade look good, and then we dump the stuff as quickly as possible.
The Nightmare make-over of the Haunted Mansion is entirely a Disneyland-driven event, part of the rather substantial holiday marketing event that the park has been doing in the last several years. Started with the Holiday version of Country Bear Jamboree (soon to be sold on eBay), and has included Small World and New Orleans Square. Since the vast majority of park visitors are locals, the gola is to make a holiday trip to Disneyland a family tradition and these special events give everyone something new to look at. And merchandise for A Nightmare Before Christmas has become fashionable and someone came up with the idea of this tie-in (okay without the merchandise tie-in and the Studio trying to play nice with Tim Burton, none of this would have happened). Disneyland departments are doing all of the work on the new show; WDI has no creative involvement. Whether it ever shows up at WDW is really up to the local management in Orlando (and I dont think they worry too much about getting crowds in winter).